Rapid ionospheric variations at high latitudes: Focusing on Greenland

The Helheim glacier, located in southeast Greenland, has more than ten campaign-type Global Positioning System (GPS) sites; data processing led to the observation of a very rapid change in the ionospheric delay. To identify the cause of these sporadic disturbances, we analyzed the slant total electron content (STEC), single-differenced STEC (SD-STEC) and scintillation proxy index called the delta phase rate (DPR). From this analysis, the abrupt change of those ionospheric indicators was attributed to the line-of-sight direction to the satellite and the temporal sequence of the event was found to be highly correlated with the geometry of the GPS sites. In addition, the disturbance based on the result of SD-STEC occurred mostly during the night, from 17 UTC through 7 UTC, and across a band spanning the east-west direction. Based on the DPR indices obtained from GPS stations distributed across all of Greenland, Iceland, and northeastern Canada, the rapid ionospheric variation was found to be correlated with the time of the day and the geomagnetic latitude of the station. The disturbance was larger at the relatively low geomagnetic latitudes at night but was more significant at higher latitudes in the daytime. These rapid ionospheric variations tended to appear in band shapes parallel to the geomagnetic field. These results allow us to attribute such disturbance observed at the Helheim glacier to aurora-related phenomena.     

电离层CT变换重建方法中的小波外推技术

电离层ＣＴ技术由于数据采集系统结构的局限性，致使所得数据不完全，伪投影中采样不均匀，频域谱值不完全若用直接Ｆｏｕｒｉｅｒ方法重建，必须对伪投影采样施以均匀化和有二维频域内进行估值。本文参照已有的Ｆｏｕｒｉｅｒ外推技术，提出小波外推技术。理论分析和重建结果都说明小波外推技术具有较多优越性。     

Response of equatorial,low- and mid-latitude F-region in the American sector during the intense geomagnetic storm on 24–25 October 2011

In this paper, we present and discuss the response of the ionospheric F-region in the American sector during the intense geomagnetic storm which occurred on 24–25 October 2011. In this investigation ionospheric sounding data obtained of 23, 24, 25, and 26 October 2011 at Puerto Rico (United States), Jicamarca (Peru), Palmas, São José dos Campos (Brazil), and Port Stanley, are presented. Also, the GPS observations obtained at 12 stations in the equatorial, low-, mid- and high-mid-latitude regions in the American sector are presented. During the fast decrease of Dst (about ∼54 nT/h between 23:00 and 01:00 UT) on the night of 24–25 October (main phase), there is a prompt penetration of electric field of magnetospheric origin resulting an unusual uplifting of the F region at equatorial stations. On the night of 24–25 October 2011 (recovery phase) equatorial, low- and mid-latitude stations show h′F variations much larger than the average variations possibly associated with traveling ionospheric disturbances (TIDs) caused by Joule heating at high latitudes. The foF2 variations at mid-latitude stations and the GPS-VTEC observations at mid- and low-latitude stations show a positive ionospheric storm on the night of 24–25 October, possibly due to changes in the large-scale wind circulation. The foF2 observations at mid-latitude station and the GPS-VTEC observations at mid- and high-mid-latitude stations show a negative ionospheric storm on the night of 24–25 October, probably associated with an increase in the density of molecular nitrogen. During the daytime on 25 October, the variations in foF2 at mid-latitude stations show large negative ionospheric storm, possibly due to changes in the O/N2 ratio. On the night of 24–25, ionospheric plasma bubbles (equatorial irregularities that extended to the low- and mid-latitude regions) are observed at equatorial, low- and mid-latitude stations. Also, on the night of 25–26, ionospheric plasma bubbles are observed at equatorial and low-latitude regions.     

Two-dimensional ionospheric total electron content map (TEC) seismo-ionospheric anomalies through image processing using principal component analysis

This paper examines China’s Wenchuan Earthquake of 12 May 2008 (UTC) (M_w = 7.9) using principal component analysis and image processing of the global ionospheric map (GIM) for the region. Transforms are conducted for 4, 8, and 9 May 2008. The GIMs are subdivided into 100 (36° in Long. and 18° in Lat.) smaller maps. The smaller maps (71 × 71 pixels) form the transform matrices of corresponding dimensions (2 × 1) through image processing. The transform allows for principle eigenvalues to be assigned to TEC anomalies for May 8 and 9. These may represent the seismo-ionospheric signature described by Zhao et al. (2008). The May 4 result shows no evidence of TEC anomalies. These results are in keeping with the findings of Liu et al. (2009). It is evident in this research that PCA could have the capacity to detect both the seismo-ionospheric signature and determine the approximate location of an earthquake’s epicenter prior to nucleation.     

HOI延迟对LEO卫星简化动力学POD的影响

通常使用无电离层(IF)线性组合(LC)消除低地球轨道(LEO)卫星简化动力学精密定轨(POD)一阶电离层延迟误差,忽略了高阶电离层(HOI)延迟误差。随着LEO卫星POD技术的发展,计算不同轨道高度的HOI延迟并探索其变化已成为进一步提高POD精度的重要手段。首先,使用国际参考电离层-2016(IRI-2016)和国际地磁参考场第13代(IGRF-13)模型,计算电离层穿刺点(IPP)位置和地磁场强度。其次,使用平滑星载GNSS数据计算电离层斜路径总电子含量(STEC)。然后,分别计算GOCE、GRACE-A和SWARM-A/B卫星的二阶和三阶电离层延迟。最后,评估了HOI延迟对LEO卫星重叠轨道分析、卫星激光测距(SLR)检核和精密科学轨道(PSO)比较结果的影响。实验结果表明：HOI延迟对LEO卫星简化动力学POD的影响大约在毫米至厘米的数量级上;HOI延迟对LEO卫星简化动力学POD外符合精度的影响分别达到0.92,0.22,0.21和0.18 mm;随着LEO卫星轨道高度的增加,HOI延迟对LEO卫星简化动力学POD的影响减小。     

轨道随机不平顺对车辆／轨道系统横向振动的影响

基于车辆－轨道耦合动力学理论，建立了车辆－轨道垂横耦合模型，利用时域数值积分法在时域仿真得到耦合系统的横向随机振动响应，再用周期图法估计出车辆、轨道横向随机响应功率谱密度，利用谱分析详细研究了轨道高低、水平、方向和轨距不平顺对车辆、轨道横向随机振动的影响。结果表明，车辆及轮轨横向表现为低频振动，主要受轨道方向和水平不平顺影响；钢轨和轨枕的横向振动频率分布很广，其低频段主要受轨道方向和水平不平顺影响，而高频段的振动主要由轨面垂向短波不平顺激发。     

Using GPS-SCINDA observations to study the correlation between scintillation,total electron content enhancement and depletions over the Kenyan region

This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S₄ index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.     

Multi-station observation of ionospheric irregularities over South Africa during strong geomagnetic storms

This paper presents results pertaining to the response of the mid-latitude ionosphere to strong geomagnetic storms that occurred from 31 March to 02 April 2001 and 07–09 September 2002. The results are based on (i) Global Positioning Systems (GPSs) derived total electron content (TEC) variations accompanying the storm, (ii) ionosonde measurements of the ionospheric electrodynamic response towards the storms and (iii) effect of storm induced travelling ionospheric disturbances (TIDs) on GPS derived TEC. Ionospheric data comprising of ionospheric TEC obtained from GPS measurements, ionograms, solar wind data obtained from Advanced Composition Explorer (ACE) and magnetic data from ground based magnetometers were used in this study. Storm induced features in vertical TEC (VTEC) have been obtained and compared with the mean VTEC of quiet days. The response of the mid-latitude ionosphere during the two storm periods examined may be characterised in terms of increased or decreased level of VTEC, wave-like structures in VTEC perturbation and sudden enhancement in hmF2 and h′F. The study reveals both positive and negative ionospheric storm effects on the ionosphere over South Africa during the two strong storm conditions. These ionospheric features have been mainly attributed to the travelling ionospheric disturbances (TIDs) as the driving mechanism for the irregularities causing the perturbations observed. TEC perturbations due to the irregularities encountered by the satellites were observed on satellites with pseudo random numbers (PRNs) 15, 17, 18 and 23 between 17:00 and 23:00 UT on 07 September 2002.     

A study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station

The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.16°N, 72.78°E; Geomagnetic: 12.90°N, 147.35°E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S₄ index_, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70–96% scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S₄ ? 0.2) and enhancement of ROTI with (ROTI ? 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.